Fix the issue of unused assignment from MIR liveness checking by chenyukang · Pull Request #149072 · rust-lang/rust

chenyukang · 2025-11-19T00:23:18Z

Fixes#148960
Fixes#148418

r? @davidtwco

cc @cjgillot

My first try on MIR related code, so it may not be the best fix.

rustbot · 2025-11-19T00:23:21Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

chenyukang · 2025-11-19T00:35:48Z

+ // If this place was dropped and has non-trivial drop,
+ // skip reporting field assignments.
+ if !is_direct && is_maybe_drop_guard {
+ continue;


this is for avoiding report unused_assign for this line of code:
https://github.com/rust-lang/rust/blob/main/library/std/src/io/mod.rs#L393

Why would there be an unused_assign for that line irrespective of whether there is a Drop impl?

If there's no subsequent code that explicitly reads g.len, this assignment appears "unused".
I think the liveness checking in MIR only knows that the entire g will be dropped, but doesn't know the drop process will read g.len. The is_direct here is used to distinguish this case and avoid false positives for field assignments that Drop may use.

In the past fields weren't checked, only whole variables, right? Why was that changed?

The indirect statement is added here:
https://github.com/chenyukang/rust/blob/4930d3e6129b17538e687f97c3b3f3e0dcaea76f/compiler/rustc_mir_transform/src/liveness.rs#L656-L661
it seems to be trying to record the access for a variable with a field:
letmut waiter_queue = .... ; ...waiter_queue.set_state_on_drop_to = ..
@cjgillot correct me if I'm wrong.

another issue which related to Drop, but this PR does not fix it:
#148418
maybe we need a more general fix.

I added a new commit for this PR to fix #148418
00f3155
I double checked the changes for testcase tests/ui/drop/or-pattern-drop-order.stderr, it's now the same with stable branch.

cjgillot

I wonder about a more principled fix. Instead of trying guess is this is a drop guard when emitting the diagnostic, should we change the dataflow analysis to make the Drop terminator mark live on interesting types? Maybe as a follow-up...

View changes since this review

davidtwco · 2025-11-22T16:16:44Z

This seems reasonable to me, even if we want a principled fix, this is useful for the time being.

@bors r+ rollup

bors · 2025-11-22T16:16:47Z

📌 Commit 00f3155 has been approved by davidtwco

It is now in the queue for this repository.

…0, r=davidtwco Fix the issue of unused assignment from MIR liveness checking Fixesrust-lang#148960Fixesrust-lang#148418 r? `@davidtwco` cc `@cjgillot` My first try on MIR related code, so it may not be the best fix.

Rollup of 9 pull requests Successful merges: - #148407 (Warn against calls which mutate an interior mutable `const`-item) - #149065 (Address annotate-snippets test differences) - #149072 (Fix the issue of unused assignment from MIR liveness checking) - #149077 (feat: Enable annotate-snippets' simd feature) - #149168 (Fix ICE when collecting opaques from trait method declarations) - #149180 (Couple of refactors to SharedEmitter) - #149185 (Handle cycles when checking impl candidates for `doc(hidden)`) - #149194 (Move safe computation out of unsafe block) - #149204 (Fix typo in HashMap performance comment) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #149072 - chenyukang:yukang-fix-unused-148960, r=davidtwco Fix the issue of unused assignment from MIR liveness checking Fixes#148960Fixes#148418 r? ``@davidtwco`` cc ``@cjgillot`` My first try on MIR related code, so it may not be the best fix.

apiraino · 2025-11-27T15:11:08Z

Beta backport accepted as per compiler team on Zulip. A backport PR will be authored by the release team at the end of the current development cycle. Backport labels handled by them.

@rustbot label +beta-accepted

[beta] backports - rustdoc: Use configured target modifiers when collecting doctests #148068 - fix(rustdoc): Color doctest errors #148834 - Fix the issue of unused assignment from MIR liveness checking #149072 - Skip unused variables warning for unreachable code #149096 - In `BTreeMap::eq`, do not compare the elements if the sizes are different. #149125 - Handle cycles when checking impl candidates for `doc(hidden)` #149185 - Generalize branch references #148395 - only the commit updating CI scripts - Change default branch references #148564 r? cuviper

Rollup of 9 pull requests Successful merges: - rust-lang/rust#148407 (Warn against calls which mutate an interior mutable `const`-item) - rust-lang/rust#149065 (Address annotate-snippets test differences) - rust-lang/rust#149072 (Fix the issue of unused assignment from MIR liveness checking) - rust-lang/rust#149077 (feat: Enable annotate-snippets' simd feature) - rust-lang/rust#149168 (Fix ICE when collecting opaques from trait method declarations) - rust-lang/rust#149180 (Couple of refactors to SharedEmitter) - rust-lang/rust#149185 (Handle cycles when checking impl candidates for `doc(hidden)`) - rust-lang/rust#149194 (Move safe computation out of unsafe block) - rust-lang/rust#149204 (Fix typo in HashMap performance comment) r? `@ghost` `@rustbot` modify labels: rollup

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 19, 2025

rustbot assigned davidtwco Nov 19, 2025

chenyukang commented Nov 19, 2025
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cjgillot reviewed Nov 19, 2025
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Comment threadcompiler/rustc_mir_transform/src/liveness.rs
Comment threadcompiler/rustc_mir_transform/src/liveness.rs Outdated

Fix the issue of unused assignment from MIR liveness checking
4930d3e

chenyukang force-pushed the yukang-fix-unused-148960 branch from 57c1dcf to 4930d3eCompare November 19, 2025 05:30

fix unused assigment issue for variable with drop, issue 148418
00f3155

chenyukang force-pushed the yukang-fix-unused-148960 branch from b7c9e8d to 00f3155Compare November 20, 2025 12:01

davidtwco approved these changes Nov 22, 2025
View reviewed changes

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 22, 2025

This was referenced Nov 22, 2025
Rollup of 10 pull requests #149220
Closed
Rollup of 9 pull requests #149222
Merged

bors merged commit cef578b into rust-lang:mainNov 22, 2025
11 checks passed

rustbot added this to the 1.93.0 milestone Nov 22, 2025

theemathas added the beta-nominated Nominated for backporting to the compiler in the beta channel. label Nov 22, 2025

dtolnay mentioned this pull request Nov 23, 2025
Suppress unused variable warning on variables used in group macro dtolnay/syn#1946
Merged

rustbot added the beta-accepted Accepted for backporting to the compiler in the beta channel. label Nov 27, 2025

cuviper mentioned this pull request Dec 3, 2025
[beta] backports #149572
Merged

cuviper modified the milestones: 1.93.0, 1.92.0 Dec 3, 2025

cuviper removed the beta-nominated Nominated for backporting to the compiler in the beta channel. label Dec 3, 2025

Mark-Simulacrum mentioned this pull request Dec 5, 2025
Wrong warning message when using ? on a expression of type Result<Infallible, Infallible> on the 1.92.0-beta.1 #148326
Closed

apiraino mentioned this pull request Dec 12, 2025
Regression unused_assignments for variable with underscore prefix captured by closure #149889
Closed

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